Gallium nitride semiconductor light emitting device having multi-quantum-well structure active layer, and semiconductor laser light source device

ABSTRACT

A gallium nitride semiconductor laser device has an active layer ( 6 ) made of a nitride semiconductor containing at least indium and gallium between an n-type cladding layer ( 5 ) and a p-type cladding layer ( 9 ). The active layer ( 6 ) is composed of two quantum well layers ( 14 ) and a barrier layer ( 15 ) interposed between the quantum well layers, and constitutes an oscillating section of the semiconductor laser device. The quantum well layers ( 14 ) and the barrier layer ( 15 ) have thicknesses of, preferably, 10 nm or less. In this semiconductor laser device, electrons and holes can be uniformly distributed in the two quantum well layers ( 14 ). In addition, electrons and holes are effectively injected into the quantum well layers from which electrons and holes have already been disappeared by recombination. Consequently, the semiconductor laser device has an excellent laser oscillation characteristic.

TECHNICAL FIELD

The present invention relates to gallium nitride semiconductor lightemitting devices such as semiconductor lasers and semiconductor diodes,and also to semiconductor laser light source devices, and moreparticularly, to a light emitting device having a multi-quantum-wellstructure active layer made of nitride semiconductor.

BACKGROUND ART

As a semiconductor material for semiconductor laser devices (LDs) andlight emitting diode devices (LEDs) having emission wavelengths within awavelength range of ultraviolet to green, gallium nitride semiconductors(GaInAlN) are used. A blue LD using such a gallium nitride semiconductoris described in, for example, Applied Physics Letters, vol. 69, No. 10,p. 1477-1479, and a sectional view of the blue LD is shown in FIG. 19.FIG. 20 is an enlarged view of part E in FIG. 19.

Referring to FIG. 19, reference numeral 101 denotes a sapphiresubstrate, 102 denotes a GaN buffer layer, 103 denotes an n-GaN contactlayer, 104 denotes an n-In_(0.05)Ga_(0.95)N layer, 105 denotes ann-Al_(0.05)Ga_(0.95)N cladding layer, 106 denotes an n-GaN guide layer,107 denotes a multi-quantum-well structure active layer composed ofIn_(0.2)Ga_(0.8)N quantum well layers and In_(0.05)Ga_(0.95)N barrierlayers, 108 denotes a p-Al_(0.2)Ga_(0.8)N layer, 109 denotes a p-GaNguide layer, 110 denotes a p-Al_(0.05)Ga_(0.95)N cladding layer, 111denotes a p-GaN contact layer, 112 denotes a p-side electrode, 113denotes an n-side electrode, and 114 denotes a SiO₂ insulating film. Inthis arrangement, as shown in FIG. 20, the multi-quantum-well structureactive layer 107 is composed of five 3 nm thick In_(0.2)Ga_(0.8)Nquantum well layers 117 and four 6 nm thick In_(0.05)Ga_(0.95)N barrierlayers 118, totally nine layers, where the quantum well layers and thebarrier layers are alternately formed.

Also, in Applied Physics Letters, vol. 69, No. 20, p. 3034-3036, thereis described a structure that the quantum well structure active layer iscomposed of alternately stacked three 4 nm thick quantum well layers andtwo 8 nm thick barrier layers, totally five layers.

Japanese Patent Laid-Open Publication HEI 8-316528 also describes a blueLD using a gallium nitride semiconductor. This prior-art blue LD alsouses a multi-quantum-well structure active layer having five or morequantum well layers, as in the case shown in FIGS. 19 and 20.

Meanwhile, a blue LED using a gallium nitride semiconductor is describedin, for example, the aforementioned Japanese Patent Laid-OpenPublication HEI 8-316528, and a sectional view of the blue LED is shownin FIG. 21. Referring to FIG. 21, reference numeral 121 denotes asapphire substrate, 122 denotes a GaN buffer layer, 123 denotes an n-GaNcontact layer, 124 denotes an n-Al_(0.3)Ga_(0.7)N second cladding layer,125 denotes an n-In_(0.01)Ga_(0.99)N first cladding layer, 126 denotes a3 nm thick In_(0.05)Ga_(0.95)N single-quantum-well structure activelayer, 127 denotes a p-In_(0.01)Ga_(0.99)N first cladding layer, 128denotes a p-Al_(0.3)Ga_(0.7)N second cladding layer, 129 denotes a p-GaNcontact layer, 130 denotes a p-side electrode, and 131 denotes an n-sideelectrode. Like this, in blue LEDs using gallium nitride semiconductors,an active layer having only one quantum well layer has been used.

The conventional blue LDs and blue LED described above, however, havehad the following problems.

Referring first to the blue LDs, the value of oscillation thresholdcurrent is as high as 100 mA or more and so needs to be largely reducedfor practical use in information processing for optical disks or thelike. Further, if the LD is used for optical disks, in order to preventdata read errors due to noise during data reading, it is necessary toinject a high-frequency current of an about 300 MHz frequency into theLD and modulate an optical output power with the same frequency. In theconventional blue LDs, however, optical output power is not modulatedeven if a high-frequency current is injected, causing a problem of dataread errors.

Referring now to blue LEDs, which indeed have been in practical use, inorder to allow blue LEDs to be used for a wider variety of applicationsincluding, for example, large color displays capable of displayingbright even at wide angles of visibility, it is desired to realize evenhigher brightness LEDs by improving optical output power.

Furthermore, conventional gallium nitride LEDs have a problem that asthe injection current increases, the peak value of emission wavelengthslargely varies. For example, in a gallium nitride blue LED, as theforward current is increased from 0.1 mA to 20 mA, the peak value ofemission wavelengths shifts by as much as 7 nm. This is particularlynoticeable in LED devices having long emission wavelengths; for example,in a gallium nitride green LED, the peak value of emission wavelengthsshifts by as much as 20 nm. When such a device is used in a colordisplay, there would occur a problem that colors of images varydepending on the brightness of the images because of the shift of thepeak wavelength.

DISCLOSURE OF THE INVENTION

In view of the above, a primary object of the present invention is tosolve the above-described problems of the gallium nitride semiconductorlight emitting devices and provide a gallium nitride semiconductor lightemitting device which makes it possible to realize a semiconductor laserdiode having satisfactory laser oscillation characteristics as well as alight emitting diode capable of yielding high optical output power.

A further object of the present invention is to provide a galliumnitride semiconductor light emitting device which makes it possible torealize a light emitting diode device free from the shift of the peakwavelength due to the injection of electric current.

A gallium nitride semiconductor light emitting device according to anembodiment of the present invention comprises a semiconductor substrate,an active layer having a quantum well structure and made of nitridesemiconductor containing at least indium and gallium, and a firstcladding layer and a second cladding layer for sandwiching the activelayer therebetween, and the active layer is composed of two quantum welllayers and one barrier layer interposed between the quantum well layers.

When this gallium nitride semiconductor light emitting device is used asa semiconductor laser device, the active layer forms an oscillatingsection of the semiconductor laser device. Besides, when a drivingcircuit for injecting an electric current into the semiconductor laserdevice is provided, a semiconductor laser light source device isrealized. Meanwhile, when the gallium nitride semiconductor lightemitting device is used as a semiconductor light emitting diode device,the active layer forms a light emitting section of the semiconductorlight emitting diode device.

In making the present invention as described above, the present inventorinvestigated in detail the causes of the aforementioned problems of theconventional devices. As a result, the following was found out.

First, with regard to blue LDs, in the InGaN material to be used for aquantum well layer, both electrons and holes have large effective massesand numerous crystal defects are present, causing the mobility of theelectrons and holes to considerably lower, so that the electrons andholes are not distributed uniformly in all the quantum well layers ofthe multi-quantum-well structure active layer. That is, electrons areinjected into only two or so of the quantum well layers on the n-typecladding layer side for electron injection, and holes are injected intoonly two or so of the quantum well layers on the p-type cladding layerside for hole injection. Accordingly, in the multi-quantum-wellstructure active layer having five or more quantum well layers, becauseof a small percentage or rate at which electrons and holes are presentin the same quantum well layer, the efficiency of light emission byrecombination of electrons and holes lowers, causing the thresholdcurrent value of laser oscillation to increase.

Also, because of the low mobility of electrons and holes as shown above,the move of electrons and holes between quantum well layers is sloweddown so that electrons and holes cannot be newly injected into thequantum well layers from which electrons and holes have already beendisappeared by recombination, and that the electrons and holes that havebeen injected into quantum well layers close to the cladding layersremain present in the same quantum well layers as they are. Accordingly,even if the injection current is modulated, the densities of electronsand holes present in the quantum well layers are not modulated. This iswhy injection of a high-frequency current does not modulate the opticaloutput power.

In the light of this finding, in the embodiment of the presentinvention, two quantum well layers are provided in the active layer madeof nitride semiconductor containing at least indium and gallium, so thatelectrons and holes are uniformly distributed in all the quantum welllayers. This realizes the improvement of the emission efficiency andhence the reduction of the oscillation threshold current value. Further,because the injection of electrons and holes into the quantum welllayers from which electrons and holes have disappeared due to theirrecombination is effectively achieved, the injection of a high-frequencycurrent successfully modulates the densities of electrons and holespresent in the quantum well layers and hence the optical output power.

For making electrons and holes uniformly distributed in all the quantumwell layers like this, because too large a layer thickness of a quantumwell layer would hinder electrons and holes from being uniformlydistributed, each of the quantum well layers preferably has a thicknessof 10 nm or less.

Likewise, because too large a layer thickness of the barrier layer wouldhinder electrons and holes from being uniformly distributed, the barrierlayer preferably has a thickness of 10 nm or less.

Meanwhile, with regard to blue LEDs, practically used devices have atendency that the optical output power comes to be saturated as thecurrent is injected more and more, as shown in FIG. 9. In theconventional blue LEDs, which have only one quantum well active layer,injected electrons and holes are both present in this one quantum welllayer, but with the increasing amount of injection, the distribution ofinjected electrons and holes spreads widely within the momentum spacebecause of the large effective masses of the electrons and holes inInGaN that forms the quantum well layer, with the result that theemission efficiency is lowered. Therefore, with the provision of the twoquantum well layers in the multi-quantum-well structure active layermade of a nitride semiconductor containing at least indium and gallium,as in the present invention, injected electrons and holes are dividedinto the two quantum well layers, by which the densities of electronsand holes present per quantum well layer are reduced. Thus, thedistribution of electrons and holes in the momentum space is reduced. Asa result of this, the tendency of saturation in the current vs. opticaloutput power characteristic has been mended, and a gallium nitride LEDdevice with high brightness attributable to improved optical outputpower has been realized.

Further, another investigation and experiment that the present inventorperformed proved that with a 4 nm or lower thickness of the barrierlayer, even if the quantum well layers are increased in number up tofour, results similar to those described above could be obtained in bothLDs and LEDs. The quantum well structure active layer of theconventional device described in the foregoing literature, “AppliedPhysics Letters, vol. 69, No. 20, p. 3034-3036”, has three quantum welllayers, but because of the large effective masses of electrons and holesof the InGaN material as well as a large thickness of the barrier layerof as much as 8 nm, the wave functions of electrons and holes hardlyoverlap between the quantum well layers. Therefore, there occur almostno moves of electrons or holes between the quantum well layers, whichhas caused nonuniform distribution of electrons and holes morenoticeably. However, it has been discovered that even if three or fourquantum well layers are provided, the wave functions of electrons andholes can be overlapped between the quantum well layers by setting thethickness of barrier layers to 4 nm or less.

It has also been found out that setting the thickness of the barrierlayer to 4 nm or less simultaneously solves the problem of peakwavelength shift due to current injection. The cause of such wavelengthshift could be considered as follows. That is, in the InGaN material,the electron-hole plasma effect is noticeable because of the largeeffective masses of electrons and holes, so that energy band ends islargely deformed due to this effect, resulting in an increased shift ofthe peak emission wavelength due to the current injection. Therefore, itcan be concluded that as a result of suppressing the electron-holeplasma effect by reducing the densities of electrons and holes perquantum well layer in such a way that the injected electrons and holesare divided uniformly into the individual quantum well layers as in thepresent invention, the wavelength shifts due to the current injectionhave also been reduced.

Further objects, features and advantages of the present-invention willbe understood from the detailed description of several embodimentsthereof which will be given below with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a semiconductor laser deviceaccording to a first embodiment of the present invention;

FIG. 2 is an enlarged sectional view of part A in FIG. 1;

FIG. 3 is a graph showing the dependence of threshold current on thenumber of quantum well layers as well as the dependence of a maximummodulation frequency of injection current capable of modulating theoptical output power on the number of quantum well layers in the firstembodiment;

FIG. 4 is a graph showing the dependence of a maximum frequency ofinjection current capable of modulating the optical output power on thethickness of the barrier layer in the first embodiment;

FIG. 5 is a circuit diagram showing a semiconductor laser device and adriving circuit according to a second embodiment of the presentinvention;

FIG. 6 is a circuit diagram showing a semiconductor laser device and adriving circuit according to a third embodiment of the presentinvention;

FIG. 7 is a sectional view showing a semiconductor light emitting diodedevice according to a fourth embodiment of the present invention;

FIG. 8 is an enlarged sectional view of part B in FIG. 7;

FIG. 9 is a graph showing current—optical output power characteristicsof the semiconductor light emitting diode device according to the fourthembodiment and of the semiconductor light emitting diode deviceaccording to the prior art, respectively;

FIG. 10 is a sectional view showing a semiconductor laser deviceaccording to a fifth embodiment of the present invention;

FIG. 11 is an enlarged sectional view showing part C of FIG. 10;

FIG. 12 is a graph showing the dependence of threshold current on thenumber of quantum well layers as well as the dependence of a maximummodulation frequency of injection current capable of modulating theoptical output power on the number of quantum well layers in the fifthembodiment;

FIG. 13 is a graph showing the dependence of a maximum frequency ofinjection current capable of modulating the optical output power on thethickness of the barrier layer in gallium nitride semiconductor laserdevices having two, three and four quantum well layers, respectively;

FIG. 14 is a circuit diagram showing a semiconductor laser device and adriving circuit according to a sixth embodiment of the presentinvention;

FIG. 15 is a circuit diagram showing a semiconductor laser device and adriving circuit according to a seventh embodiment of the presentinvention;

FIG. 16 is a sectional view showing a semiconductor light emitting diodedevice according to an eighth embodiment of the present invention;

FIG. 17 is an enlarged sectional view showing part D in FIG. 16;

FIG. 18 is a graph showing current vs. optical output powercharacteristics of the semiconductor light emitting diode deviceaccording to the eighth embodiment and the semiconductor light emittingdiode device according to the prior art, respectively;

FIG. 19 is a sectional view showing a structure example of a blue LDaccording to the prior art;

FIG. 20 is an enlarged sectional view showing part E in FIG. 19; and

FIG. 21 is a sectional view showing a structure example of a blue LEDaccording to the prior art.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinbelow, embodiments of the present invention are described withreference to the accompanying drawings.

First Embodiment

FIG. 1 is a sectional view showing a gallium nitride semiconductor laserdevice according to a first embodiment of the present invention, andFIG. 2 is an enlarged sectional view of part A of FIG. 1. In thisembodiment, a quantum well structure active layer consists of twoquantum well layers and one barrier layer disposed therebetween.

Referring to FIGS. 1 and 2, reference numeral 1 denotes a c-facesapphire substrate, 2 denotes a GaN buffer layer, 3 denotes an n-GaNcontact layer, 4 denotes an n-Al_(0.1)Ga_(0.95)N cladding layer, 5denotes an n-GaN guide layer, 6 denotes a multi-quantum-well structureactive layer consisting of two In_(0.2)Ga_(0.8)N quantum well layers 14and one In_(0.05)Ga_(0.95)N barrier layer 15, 7 denotes anAl_(0.2)Ga_(0.8)N evaporation inhibiting layer, 8 denotes a p-GaN guidelayer, 9 denotes a p-Al_(0.1)Ga_(0.9)N cladding layer, 10 denotes ap-GaN contact layer, 11 denotes a p-side electrode, 12 denotes an n-sideelectrode, and 13 denotes a SiO₂ insulating film. In FIG. 1 themulti-quantum-well structure active layer 6 formed of a plurality oflayers is depicted as if it consisted of a single layer, for the sake ofsimplification. That is the case also with FIGS. 7, 10 and 16 showingsectional views of other embodiments.

The top surface of the sapphire substrate 1 in this embodiment mayalternatively be of another orientation such as a-face, r-face andm-face. Also, not only the sapphire substrate but also a SiC substrate,a spinel substrate, a MgO substrate, a Si substrate or a GaAs substratemay be used. In particular, the SiC substrate, which is more easilycleaved as compared with the sapphire substrate, has an advantage that alaser resonator end face is easy to form by cleaving. The buffer layer 2is not limited to GaN, and may be substituted by other material such asAlN or a ternary mixed crystal AlGaN so long as the material allows agallium nitride semiconductor to be epitaxially grown thereon.

The n-type cladding layer 4 and the p-type cladding layer 9 may also beof an AlGaN ternary mixed crystal having an Al content different fromthat of n-Al_(0.1)Ga_(0.9)N. In this case, increasing the Al contentincreases energy gap difference and refractive index difference betweenthe active layer and the cladding layers, so that carriers and light canbe effectively confined in the active layer, which makes it possible tofurther reduce the oscillation threshold current and to improvetemperature characteristics. Also, decreasing the Al content whilemaintaining the confinement of carriers and light causes the mobility ofthe carriers in the cladding layers to increase, thus producing anadvantage that the device resistance of the semiconductor laser deviceis decreased. Further, alternatively these cladding layers may be madefrom a quaternary or higher mixed crystal semiconductor containing otherelements in trace amounts, and the n-type cladding layer 4 and thep-type cladding layer 9 may be different in composition of the mixedcrystal from each other.

The guide layers 5 and 8 are not limited to GaN, and may be made fromother material such as InGaN, AlGaN or other ternary mixed crystal, orInGaAlN or other quaternary mixed crystal so long as the material has anenergy gap value falling between the energy gap of the quantum welllayers of the multi-quantum-well structure active layer 6 and the energygap of the cladding layers 4, 9. Also, each guide layer does not need tobe doped all over with a donor or an acceptor, but may be partly leftnon-doped on one side closer to the multi-quantum-well structure activelayer 6, and furthermore the whole guide layer may be left non-doped. Inthis case, the carriers present in the guide layers are reduced inamount so that light absorption by free carriers is reduced. Thus,advantageously the oscillation threshold current can be further reduced.

For the two In_(0.2)Ga_(0.8)N quantum well layers 14 and the oneIn_(0.05)Ga_(0.95)N barrier layer 15 constituting the multi-quantum-wellstructure active layer 6, their compositions may be set according to anecessary laser oscillation wavelength. The In content of the quantumwell layers 14 should be increased for longer oscillation wavelengths,and the In content of the quantum well layers 14 should be decreased forshorter oscillation wavelengths. Further, the quantum well layers 14 andthe barrier layer 15 may also be made from quaternary or higher mixedcrystal semiconductor composed of InGaN ternary mixed crystal and, inaddition to this, other elements such as Al in trace amounts. Thebarrier layer 15 may also be made of GaN simply.

Next, with reference to FIGS. 1 and 2, the method for fabricating theabove gallium nitride semiconductor laser is described. Although theMOCVD (Metal Organic Chemical Vapor Deposition) method is used in thefollowing description, yet the growth method has only to be capable ofmaking GaN epitaxially grown, and other vapor phase growth method suchas MBE (Molecular Beam Epitaxy) or HDVPE (Hydride Vapor Phase Epitaxy)may be also used.

First, on a sapphire substrate 1 having the c plane as a top surface andplaced within a growth furnace, a GaN buffer layer 2 is grown to 35 nmat a growth temperature of 550° C. by using trimethyl gallium (TMG) andammonia (NH₃) as sources.

Next, with the growth temperature elevated to 1050° C., a 3 μm thickSi-doped n-GaN contact layer 3 is grown by using TMG and NH₃ as well assilane-gas (SiH₄) as source materials. Subsequently, with trimethylaluminum (TMA) added to the source materials and with the growthtemperature held at 1050° C., a Si-doped n-Al_(0.1)Ga_(0.9)N claddinglayer 4 is grown to a thickness of 0.7 μm. Subsequently, with TMAwithdrawn from the source materials and with the growth temperature heldat 1050° C., a Si-doped n-GaN guide layer 5 is grown to 0.05 μm.

Next, with the growth temperature lowered to 750° C., by using TMG, NH₃and trimethyl indium (TMI) as source materials, an In_(0.2)Ga_(0.9)Nquantum well layer (with a thickness of 5 nm) 14, an In_(0.05)Ga_(0.95)Nbarrier layer (with a thickness of 5 nm) 15, an In_(0.2)Ga_(0.8)Nquantum well layer (with a thickness of 5 nm) 14 are grown one afteranother to form a multi-quantum-well structure active layer (with atotal thickness of 15 nm) 6. Subsequently, with TMG, TMA and NH₃ used assource materials and with the growth temperature held at 750° C., aAl_(0.2)Ga_(0.8)N evaporation inhibiting layer 7 is grown to a thicknessof 10 nm.

Next, with the growth temperature elevated again to 1050° C. and withTMG and NH₃ as well as cyclopentadienyl magnesium (Cp₂Mg) used as sourcematerials, a Mg-doped p-GaN guide layer 8 is grown to a thickness of0.05 μm. Subsequently, with TMA added to the source materials and withthe growth temperature held at 1050° C., a 0.7 μm thick Mg-dopedp-Al_(0.1)Ga_(0.9)N cladding layer 9 is grown. Then, with TMA withdrawnfrom the source material and with the growth temperature held at 1050°C., a Mg-doped p-GaN contact layer 10 is grown to 0.2 μm in thickness.Thus, a gallium nitride epitaxial wafer is completed.

After these process steps, this wafer is annealed in a nitrogen gasatmosphere of 800° C., so that the Mg-doped p-type layers are lowered inresistance.

Further, by using ordinary photolithography and dry etching techniques,an etching is done from the topmost surface of the p-GaN contact layer10 until the n-GaN contact layer 3 is exposed, to obtain a 200 μm widestripe shape. Next, by using photolithography and dry etching techniquessimilar to the foregoing, the p-GaN contact layer 10 and thep-Al_(0.1)Ga_(0.9)N cladding layer 9 are etched so that the remainingp-GaN contact layer 10 has a ridge structure in a 5 μm wide stripeshape.

Subsequently, a 200 nm thick SiO₂ insulating film 13 is formed on theside faces of the ridge R and the surface of the p-type layer other thanthe ridge R. A p-side electrode 11 composed of nickel and gold is formedon the surface of this SiO₂ insulating film 13 and the p-GaN contactlayer 10, and an n-side electrode 12 composed of titanium and aluminumis formed on the surface of the n-GaN contact layer 3 exposed by theetching. Thus, a gallium nitride LD (laser diode) wafer is completed.

After these process steps, the thus obtained wafer is cleaved in adirection vertical to the ridge stripe to thereby form a laser resonatorend face, and then the wafer is divided into individual chips. Then,each chip is mounted on a stem, and the electrodes are connected withlead terminals by wire bonding. As a result, a gallium nitridesemiconductor LD device is completed.

On the blue LD device fabricated in this way, it was confirmed thatlaser characteristics of an oscillation wavelength of 430 nm and anoscillation threshold current of 40 mA can be obtained, and that anoptical output power can be enough modulated by injection of ahigh-frequency current of from 300 MHz to maximally around 1 GHz.Consequently, with the blue LD device of this embodiment, a blue LDdevice capable of preventing data read errors and usable for opticaldisks was realized.

FIG. 3 is a graph representing variations in threshold current value andmaximum modulation frequency of injection current capable of modulatingan optical output power resulting when the number of quantum well layersis changed from 1 to 5 in gallium nitride semiconductor laser devices.The semiconductor lasers are same in structure as the gallium nitridesemiconductor laser device according to the first embodiment of thepresent invention, except for the number of quantum well layers and thenumber of barrier layers, which depends on the number of quantum welllayers. As can be understood from this figure, it is only the galliumnitride semiconductor laser device according to the first embodiment ofthe present invention, in which the number of quantum well layers istwo, that has a low oscillation threshold current and has an opticaloutput power enough modulated by the injection of a high-frequencycurrent in the range of from 300 MHz to around 1 GHz at the most.

The quantum well layers 14 and the barrier layer 15, which constitutethe multi-quantum-well structure active layer 6, have both been set to alayer thickness of 5 nm in this embodiment. However, these layers arenot necessarily of the same layer thickness, but may be different fromeach other. Also, as far as the layer thickness of each of the quantumwell layers 14 and the barrier layer 15 is set to 10 nm or less foruniform injection of electrons and holes into the two quantum welllayers, similar effects can be obtained even with other layerthicknesses than the thickness used in this embodiment.

FIG. 4 is a graph representing variations in injection current maximummodulation frequency capable of modulating an optical output powerresulting when the layer thickness of the barrier layer is changed in agallium nitride semiconductor laser device in which the number ofquantum well layers is two. This semiconductor laser is similar instructure to the gallium nitride semiconductor laser device according tothe first embodiment, except that they differ in the layer thickness ofthe barrier layer. It can be understood from this figure that if thelayer thickness of the barrier layer is set to 10 nm or less, then theoptical output power can be enough modulated by the injection of ahigh-frequency current even in the range of from 300 MHz to around 1 GHzat the most. This being the case also with the quantum well layers, itwas verified that if the layer thickness of each quantum well layer isset to 10 nm or less, then the optical output power can be sufficientlymodulated by the injection of a high-frequency current in the range offrom 300 MHz to around 1 GHz at the most.

Further, the Al_(0.2)Ga_(0.8)N evaporation inhibiting layer 7 has beenformed so as to be in contact with the multi-quantum-well structureactive layer 6 in this embodiment, which arrangement is intended toinhibit the quantum well layers 14 from evaporating while the growthtemperature is increasing. Therefore, the evaporation inhibiting layer 7may be made of other materials so long as the materials protect thequantum well layers 14, and AlGaN ternary mixed crystals having other Alcompositions as well as GaN may be used for the evaporation inhibitinglayer 7. Also, this evaporation inhibiting layer 7 may be doped with Mg,in which case holes can be more easily injected from the p-GaN guidelayer 8 or the p-Al_(0.1)Ga_(0.9)N cladding layer 9, advantageously.Further, if the quantum well layers 14 have a low In content, theselayers 14 do not evaporate even without the evaporation inhibiting layer7, and so omission of the evaporation inhibiting layer 7 will not affectthe characteristics of the gallium nitride semiconductor laser device ofthis embodiment.

Whereas a ridge stripe structure is formed to achieve the contraction ornarrowing of injection current in this embodiment, it is also possibleto use other current contracting techniques such as an electrode stripestructure. Also, whereas a laser resonator end face is formed bycleaving in this embodiment, the end face can also be formed by dryetching in cases that the sapphire substrate is too hard to cleave.

Further, in this embodiment, in which sapphire that is an insulatingmaterial is used as a substrate, an n-side electrode 12 is formed on thetop surface of the n-GaN contact layer 3 exposed by etching. However, ifSiC, Si, GaAs or the like having n-type electrical conductivity is usedas a substrate, then the n-side electrode 12 may be formed on the rearsurface of this substrate. Besides, the p-type structure and the n-typestructure may be reversed.

Second Embodiment

FIG. 5 is a circuit diagram showing a semiconductor laser device with adriving circuit according to a second embodiment of the presentinvention. A semiconductor laser device 16 shown in FIG. 5 is a galliumnitride semiconductor laser device, which has two quantum well layers,obtained by the first embodiment of the present invention. Ahigh-frequency driving circuit 17 is made up of ordinary semiconductorparts, and is intended to modulate an injection current to thesemiconductor laser device 16 at a high frequency and to therebymodulate its optical output power. In this embodiment, the modulationfrequency of the injection current was set to 300 MHz. The galliumnitride semiconductor laser device according to the first embodiment hasrevealed that its optical output power is modulatable even by a maximuminjection current modulation frequency of higher than 1 GHz, and theoptical output power was enough modulated by a frequency of 300 MHz.When the present embodiment was used as a light source for an opticaldisk, by virtue of sufficiently modulated optical output power of thesemiconductor laser, the laser light coherence was able to be lowered sothat noise due to return light from the disk surface was able to bereduced. As a result, it became possible to read data from the opticaldisk without errors.

The modulation frequency of the injection current was set to 300 MHz inthis embodiment. However, other modulation frequencies up to a maximumfrequency of around 1 GHz may be used to drive the nitride semiconductorlaser so long as the modulation frequency allows the noise due to returnlight of laser light from the disk surface to be reduced by lowering thecoherence of the laser light.

Third Embodiment

FIG. 6 is a circuit diagram showing a semiconductor laser device with adriving circuit according to a third embodiment of the presentinvention. For a semiconductor laser device 18 shown in FIG. 6, thegallium nitride semiconductor laser device with two quantum well layersobtained by the first embodiment of the present-invention is used, butis adjusted in the stripe width w (see FIG. 1) in the formation of theridge structure as well as in the depth for the etching of thep-Al_(0.1)Ga_(0.9)N cladding layer 9 so that the semiconductor laserdevice 18 is a self-oscillating semiconductor laser in which the opticaloutput power is modulated even by the injection of an unmodulatedconstant current. In this example, the stripe width w was set to 3 μm,the film thickness d (see FIG. 1) of the p-Al_(0.1)Ga_(0.9)N claddinglayer 9 left in the etching process was set to 0.2 μm. The stripe widthand the film thickness left in etching are not limited to the values ofthis concrete example, and have only to fall within a range of 1 to 5 μmand a range of 0.05 to 0.5 μm, respectively. The optical output powermodulation frequency for the self-oscillating gallium nitridesemiconductor laser device fabricated in this way was 800 MHz.

By virtue of the arrangement that two quantum well layers are provided,the gallium nitride semiconductor laser device according to the thirdembodiment is susceptible to modulation of the densities of electronsand holes present within the quantum well layers. Thus, it is easy tofabricate a self-oscillating semiconductor laser in which an opticaloutput power is modulated not only by modulating the densities ofelectrons and holes with modulation of the injection current but also bymodulating the densities of electrons and holes even with the injectionof an unmodulated constant current, thus making it possible to modulatethe optical output power at higher frequencies.

A constant current driving circuit 19 is implemented by using ordinarysemiconductor parts and is intended to inject a constant current to thesemiconductor laser. When this embodiment was used as a light source foran optical disk, by virtue of sufficiently modulated optical outputpower of the semiconductor laser, the laser light coherence was able tobe lowered so that noise due to return light from the disk surface wasable to be reduced. As a result, it became possible to read data fromthe optical disk without errors.

The gallium nitride semiconductor laser device 18 used in the thirdembodiment is a self-oscillating semiconductor laser obtained byadjusting both the stripe width w in forming the ridge structure and thedepth to which the p-Al_(0.1)Ga_(0.9)N cladding layer 9 is etched.Alternatively, the self-oscillating semiconductor laser may be obtainedby providing a saturable absorption layer (not shown) near the activelayer, as done in ordinary GaAs semiconductor lasers or the like.

Fourth Embodiment

FIG. 7 is a sectional view showing a gallium nitride semiconductor LEDdevice according to a fourth embodiment of the present invention, andFIG. 8 is an enlarged sectional view of part B of FIG. 7.

In these figures, reference numeral 21 denotes a c-face sapphiresubstrate, 22 denotes a GaN buffer layer, 23 denotes an n-GaN contactlayer, 24 denotes an n-Al_(0.1)Ga_(0.9)N cladding layer, 25 denotes ann-GaN guide layer, 26 denotes a multi-quantum-well structure activelayer consisting of two In_(0.2)Ga_(0.8)N quantum well layers 34 and oneIn_(0.05)Ga_(0.95)N barrier layer 35, 27 denotes an Al_(0.2)Ga_(0.8)Nevaporation inhibiting layer, 28 denotes a p-GaN guide layer, 29 denotesa p-Al_(0.1)Ga_(0.9)N cladding layer, 30 denotes a p-GaN contact layer,31 denotes a p-side electrode, and 32 denotes an n-side electrode.

The top surface of the sapphire substrate 21 in this embodiment mayalternatively be of another orientation such as a-face, r-face andm-face. Also, the sapphire substrate but also a SiC substrate, a spinelsubstrate, a MgO substrate, or a Si substrate may be used. Inparticular, the SiC substrate, which is more easily cleaved as comparedwith the sapphire substrate, has an advantage that division into chipsof the LED device is easy. The buffer layer 22 is not limited to GaN,and may be substituted by other material such as AlN or a ternary mixedcrystal AlGaN so long as the material allows a gallium nitridesemiconductor to be epitaxially grown thereon.

The n-type cladding layer 24 and the p-type cladding layer 29 may alsobe of any AlGaN ternary mixed crystal having an Al content differentfrom that of n-Al_(0.1)Ga_(0.9)N, or simply of GaN. Increasing the Alcontent increases energy gap difference between the active layer and thecladding layers, so that carriers can be effectively confined in theactive layer, which makes it possible to improve temperaturecharacteristics. On the other hand, decreasing the Al content whilemaintaining the confinement of carriers causes the mobility of thecarriers in the cladding layers to increase, thus producing an advantagethat the device resistance of the light emitting diode is decreased.Further, alternatively these cladding layers may be made from aquaternary or higher mixed crystal semiconductor containing otherelements in trace amounts, and the n-type cladding layer 24 and thep-type cladding layer 29 may be different in composition of the mixedcrystal from each other.

The guide layers 25 and 28 are not limited to GaN, and may be made fromother material such as InGaN, AlGaN or other ternary mixed crystal, orInGaAlN or other quaternary mixed crystal so long as the material has anenergy gap value falling between the energy gap of the quantum welllayers of the multi-quantum-well structure active layer 26 and theenergy gap of the cladding layers 24, 29. Also, each guide layer doesnot need to be doped all over with a donor or an acceptor, but may bepartly left non-doped on one side closer to the multi-quantum-wellstructure active layer 26, and furthermore the whole guide layer may beleft non-doped. In this case, the carriers present in the guide layersare reduced in amount so that light absorption by free carriers isreduced. Thus, advantageously the oscillation threshold current can befurther reduced, resulting in better output power. The guide layers 25and 28 advantageously facilitate injection of electrons and holes intothe multi-quantum-well structure active layer 26 from the n- andp-cladding layers 24 and 29, respectively. Those guide layers 25 and 28,however, can be dispensed with because the LED characteristics are notseriously affected or deteriorated by the dispensation of the guidelayers 25 and 28.

For the two In_(0.2)Ga_(0.8)N quantum well layers 34 and the oneIn_(0.05)Ga_(0.95)N barrier layer 35 constituting the multi-quantum-wellstructure active layer 26, their compositions may be set according to anecessary light emission wavelength. The In content of the quantum welllayers 34 should be increased for longer emission wavelengths, and theIn content of the quantum well layers 34 should be decreased for shorteremission wavelengths. Further, the quantum well layers 34 and thebarrier layer 35 may also be made from quaternary or higher mixedcrystal semiconductor composed of InGaN ternary mixed crystal and, inaddition to this, other elements such as Al in trace amounts.

Next, with reference to FIGS. 7 and 8, the method for fabricating theabove gallium nitride semiconductor LED is described. Although the MOCVD(Metal Organic Chemical Vapor Deposition) method is used in thefollowing description, yet the growth method has only to be capable ofmaking GaN epitaxially grown, and other vapor phase growth method suchas MBE (Molecular Beam Epitaxy) or HDVPE (Hydride Vapor Phase Epitaxy)may be also used.

First, on a sapphire substrate 21 having the c plane as a top surfaceand placed within a growth furnace, a GaN buffer layer 22 is grown to 35nm at a growth temperature of 550° C. by using TMG and NH₃ as sourcematerials.

Next, with the growth temperature elevated to 1050° C., a 3 μm thickSi-doped n-GaN contact layer 23 is grown by using TMG and NH₃ as well asSiH₄ as source materials. Subsequently, with TMA added to the sourcematerials and with the growth temperature held at 1050° C., a Si-dopedn-Al_(0.1)Ga_(0.9)N cladding layer 24 is grown to a thickness of 0.3 μm.Subsequently, with TMA withdrawn from the source materials and with thegrowth temperature held at 1050° C., a Si-doped n-GaN guide layer 25 isgrown to 0.05 μm.

Next, with the growth temperature lowered to 750° C., by using TMG, NH₃and TMI as source materials, an In_(0.2)Ga_(0.8)N quantum well layer(with a thickness of 3 nm) 34, an In_(0.5)Ga_(0.95)N barrier layer (witha thickness of 5 nm) 35, an In_(0.2)Ga_(0.8)N quantum well layer (with athickness of 3 nm) 34 are sequentially grown one after another to form amulti-quantum-well structure active layer (with a total thickness of 11nm) 26. Subsequently, with TMG, TMA and NH₃ used as source materials andwith the growth temperature held at 750° C., an Al_(0.2)Ga_(0.8)Nevaporation inhibiting layer 27 is grown to a thickness of 10 nm.

Next, with the growth temperature elevated again to 1050° C. and withTMG and NH₃ as well as cyclopentadienyl magnesium used as sourcematerials, a Mg-doped p-GaN guide layer 28 is grown to a thickness of0.05 aim. Subsequently, with TMA added to the source materials and withthe growth temperature held at 1050° C., a 0.3 μm thick Mg-dopedp-Al_(0.2)Ga_(0.8)N cladding layer 29 is grown. Then, with TMA withdrawnfrom the source material and with the growth temperature held at 1050°C., a Mg-doped p-GaN contact layer 30 is grown to 0.2 μm in thickness.Thus, a gallium nitride epitaxial wafer is completed.

After these process steps, this wafer is annealed in a nitrogen gasatmosphere of 800° C., so that the Mg-doped p-type layers are lowered inresistance.

Further, by using ordinary photolithography and dry etching techniques,an etching is done at predetermined regions from the topmost surface ofthe p-GaN contact layer 30 until the n-GaN contact layer 23 is exposed,for the fabrication of the LED.

Subsequently, a p-side electrode 31 composed of nickel and gold isformed on the surface of the p-GaN contact layer 30, and an n-sideelectrode 32 composed of titanium and aluminum is formed on the surfaceof the n-GaN contact layer 23 exposed by the etching. Thus, a galliumnitride LED wafer is completed.

After these process steps, the thus obtained wafer is divided intoindividual chips. Then, each chip is mounted on a stem, and theelectrodes are connected with lead terminals by wire bonding. As aresult, a gallium nitride semiconductor LED device is completed.

The blue LED device fabricated in this way had light emittingcharacteristics of a peak emission wavelength of 430 nm and an outputpower of 6 mW for a forward current of 20 mA. Also, as obvious from FIG.9, the output power was not saturated even at a large injection current,which proves that the current − optical output power characteristic wasimproved, as compared with the conventional LED device.

The quantum well layers 34 and the barrier layer 35, which constitutethe multi-quantum-well structure active layer 26, have a layer thicknessof 3 nm and a layer thickness of 5 nm, respectively, in this embodiment.However, as far as the layer thickness of each of the quantum welllayers 34 and the barrier layer 35 is set to 10 nm or less for uniforminjection of electrons and holes into the two quantum well layers,similar effects can be obtained even with other layer thicknesses.

Further, the Al_(0.2)Ga_(0.8)N evaporation inhibiting layer 27 is formedso as to be in contact with the multi-quantum-well structure activelayer 26 in this embodiment, which arrangement is intended to inhibitthe quantum well layers 34 from evaporating while the growth temperatureis increasing. Therefore, the evaporation inhibiting layer 27 may bemade of other materials so long as the materials protect the quantumwell layers 34, and an AlGaN ternary mixed crystal having other Alcontent or simply GaN may be used for the evaporation inhibiting layer27. Also, this evaporation inhibiting layer 27 may be doped with Mg, inwhich case, advantageously, holes can be more easily injected from thep-GaN guide layer 28 or the p-Al_(0.1)Ga_(0.9)N cladding layer 29.Further, if the quantum well layers 34 have a low In content, theselayers 34 do not evaporate even without the evaporation inhibiting layer27, and so omission of the evaporation inhibiting layer 27 would notaffect the characteristics of the gallium nitride semiconductor LEDdevice of this embodiment.

Fifth Embodiment

FIG. 10 is a sectional view showing a gallium nitride semiconductorlaser device according to a fifth embodiment of the present invention,and FIG. 11 is an enlarged sectional view of part C of FIG. 10. Thisgallium nitride semiconductor laser device is same as the galliumnitride semiconductor laser device according to the first embodiment,except for the structure of a multi-quantum-well structure active layer46. Therefore, for this gallium nitride semiconductor laser device, thesame reference numerals as used in FIGS. 10 and 11 are used and detaileddescription of the layers is omitted except for the multi-quantum-wellstructure active layer 46. It is needless to say that various changes,modifications and alternatives described in the first embodiment areapplicable also to this embodiment and that similar effects are obtainedtherefrom.

In this embodiment, the multi-quantum-well structure active layer 46consists of four In_(0.2)Ga_(0.8)N quantum well layers 54 and threeIn_(0.5)Ga_(0.95)N barrier layers 55, which are alternately stacked. Thethickness of each barrier layer 55 is 4 nm or less.

For the four In_(0.2)Ga_(0.98)N quantum well layers 54 and the threeIn_(0.05)Ga_(0.95)N barrier layers 55, which constitute themulti-quantum-well structure active layer 46, their compositions may beset according to a necessary laser oscillation wavelength. The Incontent of the quantum well layers 54 should be increased for longeroscillation wavelengths and decreased for shorter oscillationwavelengths. Further, the quantum well layers 54 and the barrier layers55 may also be made of a quaternary or higher mixed crystalsemiconductor composed of InGaN ternary mixed crystal and, in additionto this, other elements such as Al in trace amounts. Alternatively, thebarrier layers 55 may be made of GaN simply. Further, the number of thequantum well layers 54 may be either two or three, instead of four.However, the layer thickness of each barrier layer 55 should be set to 4nm or less, regardless of material and number of the quantum well layersso that there occur overlaps of the wave functions of electrons andholes between the quantum well layers.

Next, with reference to FIGS. 10 and 11, the method for fabricating theabove gallium nitride semiconductor laser is described. Although theMOCVD (Metal Organic Chemical Vapor Deposition) method is used in thefollowing description, yet the growth method has only to be capable ofmaking GaN epitaxially grown, and other vapor phase growth method suchas MBE (Molecular Beam Epitaxy) or HDVPE (Hydride Vapor Phase Epitaxy)may be also used.

First, on a sapphire substrate 1 having the c plane as a top surface andplaced within a growth furnace, a GaN buffer layer 2 is grown to 35 nmat a growth temperature of 550° C. by using trimethyl gallium (TMG) andammonia (NH₃) as source materials.

Next, with the growth temperature elevated to 1050° C., a 3 μm thickSi-doped n-GaN contact layer 3 is grown by using TMG and NH₃ as well assilane gas (SiH₄) as source materials. Subsequently, with trimethylaluminum (TMA) added to the source materials and with the growthtemperature held at 1050° C., a Si-doped n-Al_(0.1)Ga_(0.9)N claddinglayer 4 is grown to a thickness of 0.7 μm. Subsequently, with TMAwithdrawn from the source materials and with the growth temperature heldat 1050° C., a Si-doped n-GaN guide layer 5 is grown to 0.05 μm.

Next, with the growth temperature lowered to 750° C., by using TMG, NH₃and trimethyl indium (TMI) as source materials, alternate growth of anIn_(0.2)Ga_(0.8)N quantum well layer (with a thickness of 3 nm) 54 andan In_(0.05)Ga_(0.95)N barrier layer (with a thickness of 2 nm) 55 isrepeated three times and finally one more In_(0.2)Ga_(0.08)N quantumwell layer (with a thickness of 3 nm) 54 is grown, whereby amulti-quantum-well structure active layer (with a total thickness of 18nm) 46 is completed. Subsequently, with TMG, TMA and NH₃ used as sourcematerials and with the growth temperature held at 750° C., aAl_(0.2)Ga_(0.8)N evaporation inhibiting layer 7 is grown to a thicknessof 10 nm.

Next, with the growth temperature elevated again to 1050° C. and withTMG and NH₃ as well as cyclopentadienyl magnesium (Cp₂Mg) used as sourcematerials, a Mg-doped p-GaN guide layer 8 is grown to a thickness of0.05 μm. Subsequently, with TMA added to the source materials and withthe growth temperature held at 1050° C., a 0.7 μm thick Mg-dopedp-Al_(0.1)Ga_(0.9)N cladding layer 9 is grown. Then, with TMA withdrawnfrom the source material and with the growth temperature held at 1050°C., a Mg-doped p-GaN contact layer 10 is grown to 0.2 μm in thickness.Thus, a gallium nitride epitaxial wafer is completed.

After these process steps, this wafer is annealed in a nitrogen gasatmosphere of 800° C., so that the Mg-doped p-type layers are lowered inresistance.

Further, by using ordinary photolithography and dry etching techniques,an etching is done from the topmost surface of the p-GaN contact layer10 until the n-GaN contact layer 3 is exposed, to obtain a 200 μm widestripe shape. Next, by using photolithography and dry etching techniquessimilar to the foregoing, the p-GaN contact layer 10 and thep-Al_(0.1)Ga_(0.9)N cladding layer 9 are etched so that the remainingp-GaN contact layer 10 has a ridge structure in a 5 pt wide stripeshape.

Subsequently, a 200 nm thick SiO₂ insulating film 13 is formed on theside faces of the ridge R and the surface of the p-type layer other thanthe ridge R. A p-side electrode 11 composed of nickel and gold is formedon the surface of this SiO₂ insulating film 13 and the p-GaN contactlayer 10, and an n-side electrode 12 composed of titanium and aluminumis formed on the surface of the n-GaN contact layer 3 exposed by theetching. Thus, a gallium nitride LD (laser diode) wafer is completed.

After these process steps, the thus obtained wafer is cleaved in adirection vertical to the ridge stripe to thereby form a laser resonatorend face, and then the wafer is divided into individual chips. Then,each chip is mounted on a stem, and the electrodes are connected withlead terminals by wire bonding. As a result, a gallium nitridesemiconductor LD device is completed.

With respect to the blue LD device fabricated in this way, it wasverified that laser characteristics of an oscillation wavelength of 430nm and an oscillation threshold current of 40 mA can be obtained, andthat an optical output power can be modulated by injection of ahigh-frequency current of from 300 MHz to maximally around 1 GHz.Consequently, with the blue LD device of this embodiment, a blue LDdevice capable of preventing data read errors and usable for opticaldisks was realized.

FIG. 12 is a graph representing variations in threshold current valueand maximum modulation frequency of injection current capable ofmodulating an optical output power resulting when the number of quantumwell layers is changed from 1 to 5 in gallium nitride semiconductorlaser devices. The semiconductor lasers are same in structure as thegallium nitride semiconductor laser device according to the fifthembodiment of the present invention, except for the number of quantumwell layers and the number of barrier layers, which depends on thenumber of quantum well layers. As can be understood from this figure, itis only the gallium nitride semiconductor laser devices according to thepresent invention, in which the number of quantum well layers is two tofour, that have a low oscillation threshold current and have an opticaloutput power enough modulated by the injection of a high-frequencycurrent in the range of from 300 MHz to around 1 GHz.

The quantum well layers 54 and the barrier layers 55, which constitutethe multi-quantum-well structure active layer 6, have both been set to alayer thickness of 3 nm and 2 nm, respectively, in this embodiment.However, as far as the layer thickness of each of the quantum welllayers 54 and the layer thickness of each of the barrier layers 55 areset to 10 nm or less and 0.4 nm or less, respectively, for uniforminjection of electrons and holes into each of the quantum well layers,similar effects can be obtained even with other layer thicknesses. FIG.13 is a graph representing variations in injection current maximummodulation frequency capable of modulating an optical output power whenthe layer thickness of the barrier layer is changed in gallium nitridesemiconductor laser devices in which the number of quantum well layersis two, three, and four, respectively. These semiconductor lasers aresimilar in structure to the gallium nitride semiconductor laser deviceaccording to the fifth embodiment, except that they differ in the layerthickness of the barrier layers. It can be understood from this figurethat if the layer thickness of the barrier layers is set to 4 nm orless, then the optical output power can be enough modulated by theinjection of a high-frequency current in the range of from 300 MHz toaround 1 GHz. It has also been verified that if the layer thickness ofeach quantum well layer is set to 10 nm or less, then the optical outputpower can be sufficiently modulated by the injection of a high-frequencycurrent of even around 1 GHz.

Sixth Embodiment

FIG. 14 is a circuit diagram showing a semiconductor laser device with adriving circuit according to a sixth embodiment of the presentinvention. A semiconductor laser device 66 shown in FIG. 14 is a galliumnitride semiconductor laser device, which has four quantum well layers,obtained by the fifth embodiment of the present invention. Ahigh-frequency driving circuit 17 has a construction similar to thedriving circuit used in the second embodiment, and is intended tomodulate an injection current to the semiconductor laser device 66 at ahigh frequency and to thereby modulate its optical output power. In thisembodiment, the modulation frequency of the injection current was set to300 MHz. The gallium nitride semiconductor laser device according to thefifth embodiment has revealed that its optical output power ismodulatable even by a maximum injection current modulation frequency ofhigher than 1 GHz, and the optical output power was enough modulated bya frequency of 300 MHz. When this embodiment was used as a light sourcefor an optical disk, by virtue of sufficiently modulated optical outputpower of the semiconductor laser, the laser light coherence was able tobe lowered so that noise due to return light from the disk surface wasable to be reduced. As a result, it became possible to read data fromthe optical disk without errors.

The modulation frequency of the injection current was set to 300 MHz inthis embodiment. However, other modulation frequencies in a range offrom 300 MHz to a maximum frequency of around 1 GHz may be used to drivethe nitride semiconductor laser so long as the modulation frequencyallows the noise due to return light of laser light from the disksurface to be reduced by lowering the coherence of the laser light.

Seventh Embodiment

FIG. 15 is a circuit diagram showing a semiconductor laser device with adriving circuit according to a seventh embodiment of the presentinvention. For a semiconductor laser device 68 shown in FIG. 15, thegallium nitride semiconductor laser device with four quantum well layersobtained by the fifth embodiment of the present invention is used, butis adjusted in the stripe width for formation of the ridge structure aswell as in the depth of etching of the p-Al_(0.1)Ga_(0.9)N claddinglayer 9 so that the semiconductor laser device 68 is a self-oscillatingsemiconductor laser in which the optical output power is modulated evenby the injection of an unmodulated constant current. In this example,the stripe width was set to 3 am, and the film thickness of thep-Al_(0.1)Ga_(0.9)N cladding layer 9 left in the etching process was setto 0.2 μm. The stripe width and the film thickness are not limited tothe values of this example, and have only to fall within a range of 1 μmto 5 μm and a range of 0.05 μm to 0.5 μm, respectively. The opticaloutput power modulation frequency for the self-oscillating galliumnitride semiconductor laser device fabricated in this way was 800 MHz.

By virtue of the arrangement that two to four quantum well layers areprovided and that the thickness of the barrier layers is 4 nm or less,the gallium nitride semiconductor laser device according to the presentinvention is susceptible to modulation of the densities of electrons andholes present within the quantum well layers. Thus, it is easy tofabricate a self-oscillating semiconductor laser in which an opticaloutput power is modulated not only by modulating the densities ofelectrons and holes with modulation of the injection current but also bymodulating the densities of electrons and holes even with the injectionof an unmodulated constant current, thus making it possible to modulatethe optical output power at higher frequencies.

A constant current driving circuit 19, which is similar to the drivingcircuit used in the third embodiment, is intended to inject a constantcurrent to the semiconductor laser. When this embodiment was used as alight source for an optical disk, by virtue of sufficiently modulatedoptical output power of the semiconductor laser, the laser lightcoherence was able to be lowered so that noise due to return light fromthe disk surface was able to be reduced. As a result, it became possibleto read data from the optical disk without errors.

The gallium nitride semiconductor laser device 18 used in the seventhembodiment is a self-oscillating semiconductor laser obtained byadjusting both the stripe width for forming the ridge structure and thedepth to which the p-Al_(0.1)Ga_(0.9)N cladding layer 9 is etched.Alternatively, the self-oscillating semiconductor laser may be obtainedby providing a saturable absorption layer (not shown) near the activelayer, as done in ordinary-GaAs semiconductor lasers or the like.

Eighth Embodiment

FIG. 16 is a sectional view showing a gallium nitride semiconductor LEDdevice according to an eighth embodiment of the present invention, andFIG. 17 is an enlarged sectional view of part D of FIG. 16. This galliumnitride semiconductor LED device is same as the gallium nitridesemiconductor LED device according to the fourth embodiment, except forthe structure of a multi-quantum-well structure active layer 76.Therefore, for this gallium nitride semiconductor laser device, the samereference numerals as used in FIGS. 7 and 8 are used and detaileddescription of the layers is omitted except for the multi-quantum-wellstructure active layer 76. It is needless to say that various changes,modifications and alternatives described in connection with the fourthembodiment are applicable also to this embodiment and that similareffects are obtained therefrom.

In this embodiment, the multi-quantum-well structure active layer 76consists of three In_(0.2)Ga_(0.8)N quantum well layers 84 and twoIn_(0.05)Ga_(0.95)N barrier layers 85, which are alternately stacked.The thickness of each barrier layer 85 is 4 nm or less.

For the three In_(0.2)Ga_(0.95) N quantum well layers 84 and the twoIn_(0.05)Ga_(0.95)N barrier layers 85, which constitute themulti-quantum-well structure active layer 76, their compositions may beset according to a necessary light emission wavelength. The In contentof the quantum well layers 84 should be increased for longer emissionwavelengths and decreased for shorter emission wavelengths. Further, thequantum well layers 84 and the barrier layers 85 may also be made of aquaternary or higher mixed crystal semiconductor composed of a ternarymixed crystal of InGaN and, in addition to this, other elements such asAl in trace amounts. Alternatively, the barrier layers 85 may be made ofGaN simply. Further, the number of the quantum well layers 84 may beeither two or four, instead of three. However, the layer thickness ofeach barrier layer 85 should be set to 4 nm or less, regardless ofmaterial and number of the quantum well layers so that there occuroverlaps of the wave functions of electrons and holes between thequantum well layers.

Next, with reference to FIGS. 16 and 17, the method of fabricating theabove gallium nitride semiconductor LED is described. Although the MOCVD(Metal Organic Chemical Vapor Deposition) method is used in thefollowing description, yet the growth method has only to be capable ofmaking GaN epitaxially grown, and other vapor phase growth method suchas MBE (Molecular Beam Epitaxy) or HDVPE (Hydride Vapor Phase Epitaxy)may be also used.

First, on a sapphire substrate 21 having the c-plane as a top surfaceand placed within a growth furnace, a GaN buffer layer 22 is grown to 35nm at a growth temperature of 550° C. by using TMG and NH₃ as sourcematerials.

Next, with the growth temperature elevated to 1050° C., a 3 μm thickSi-doped n-GaN contact layer 23 is grown by using TMG and NH₃ as well asSiH₄ as source materials. Subsequently, with TMA added to the sourcematerials and with the growth temperature held at 1050° C., a Si-dopedn-Al_(0.1)Ga_(0.9)N cladding layer 24 is grown to a thickness of 0.3 μm.Subsequently, with TMA withdrawn from the source materials and with thegrowth temperature held at 1050° C., a Si-doped n-GaN guide layer 25 isgrown to 0.05 μm.

Next, with the growth temperature lowered to 750° C., by using TMG, NH₃and TMI as source materials, alternate growth of an In_(0.2)Ga_(0.8)Nquantum well layer (with a thickness of 3 nm) 84 and anIn_(0.05)Ga_(0.95)N barrier layer (with a thickness of 4 nm) 85 isrepeated twice and then one more In_(0.2)Ga_(0.8)N quantum well layer(with a thickness of 3 nm) 84 is grown, whereby a multi-quantum-wellstructure active layer (with a total thickness of 17 nm) 76 iscompleted. Subsequently, with TMG, TMA and NH₃ used as source materialsand with the growth temperature held at 750° C., an Al_(0.2)Ga_(0.8)Nevaporation inhibiting layer 27 is grown to a thickness of 10 nm.

Next, with the growth temperature elevated again to 1050° C. and withTMG and NH₃ as well as Cp₂Mg (cyclopentadienyl magnesium) used as sourcematerials, a Mg-doped p-GaN guide layer 28 is grown to a thickness of0.05 μm. Subsequently, with TMA added to the source materials and withthe growth temperature held at 1050° C., a 0.3 m thick Mg-dopedp-Al_(0.1)Ga_(0.9)N cladding layer 29 is grown. Then, with TMA withdrawnfrom the source material and with the growth temperature held at 1050°C., a Mg-doped p-GaN contact layer 30 is grown to 0.2 μm in thickness.Thus, a gallium nitride epitaxial wafer is completed.

After these process steps, this wafer is annealed in a nitrogen gasatmosphere of 800° C., so that the Mg-doped p-type layers are lowered inresistance.

Further, by using ordinary photolithography and dry etching techniques,an etching is done at predetermined regions from the topmost surface ofthe p-GaN contact layer 30 until the n-GaN contact layer 23 is exposed,for the fabrication of the LED device.

Subsequently, a p-side electrode 31 composed of nickel and gold isformed on the surface of the p-GaN contact layer 30, and an n-sideelectrode 32 composed of titanium and aluminum is formed on the surfaceof the n-GaN contact layer 23 exposed by the etching. Thus, a galliumnitride LED wafer is completed.

After these process steps, the thus obtained wafer is divided intoindividual chips. Then, each chip is mounted on a stem, and theelectrodes are connected with lead terminals by wire bonding. As aresult, a gallium nitride semiconductor LED device is completed.

The blue LED device fabricated in this way had light emittingcharacteristics of a peak emission wavelength of 430 nm and an outputpower of 6 mW for a forward current of 20 mA. Also, as obvious from FIG.18, the output power was not saturated even at a large injectioncurrent, which proves that the current − optical output powercharacteristic was improved, as compared with the conventional LEDdevice. In addition, the wavelength shift due to the current injection,which is 7 nm with the conventional blue LED device, was reduced to 2 umaccording to the present invention.

The quantum well layers 84 and the barrier layers 85, which constitutethe multi-quantum-well structure active layer 76, have a layer thicknessof 3 nm and a layer thickness of 4 nm, respectively, in this embodiment.However, as far as the quantum well layers 84 and the barrier layers 85have thicknesses of 10 nm or less and 4 nm or less, respectively, forthe uniform injection of electrons and holes into each of the quantumwell layers, similar effects can be obtained even with other layerthicknesses.

INDUSTRIAL APPLICABILITY

The gallium nitride semiconductor light emitting device of the presentinvention is used as a semiconductor laser device for processinginformation in optical disks or the like, and a semiconductor lightemitting diode device for large-size color display devices or the like.The gallium nitride semiconductor light emitting device, when combinedwith a driving circuit for injecting an electric current into asemiconductor laser device, is usable also as a semiconductor laserlight source device for, for example, data reading from an optical disk.

1-15. (canceled)
 16. A gallium nitride semiconductor laser device havingemission wavelengths within a band corresponding to ultraviolet to greenand having a ridge structure, comprising: an active layer having aquantum well structure and made of nitride semiconductor containing atleast indium and gallium, said active layer consisting of two quantumwell layers and one barrier layer interposed between the quantum welllayers; and an n-type cladding layer and a p-type cladding layer betweenwhich the active layer is disposed, said p-type cladding layer formingat least part of the ridge structure; wherein each quantum well layerhas a layer thickness of 10 nm or less.
 17. The gallium nitridesemiconductor laser device according to claim 16, wherein the ridgestructure has a width of about 1 μm to 5 μm.
 18. The gallium nitridesemiconductor laser device according to claim 16, wherein said p-typecladding layer has a ridge portion and a planar portion on oppositesides of the ridge portion, and the planar portion has a film thicknessof 0.05 μm to 0.5 μm.
 19. The gallium nitride semiconductor laser deviceaccording to claim 16, wherein each of the quantum well layers haselectrons and holes uniformly distributed therein.
 20. The galliumnitride semiconductor laser device according to claim 16, wherein thesemiconductor laser device is a self-oscillating semiconductor laserdevice.
 21. The gallium nitride semiconductor laser device according toclaim 16, further comprising a driving circuit for injecting an electriccurrent into the semiconductor laser device.
 22. The gallium nitridesemiconductor laser device according to claim 21, wherein the electriccurrent is a modulated current and a modulation frequency of the currentis 300 MHz or more.
 23. The gallium nitride semiconductor laser deviceaccording to claim 16, wherein said laser device generates a modulatedoptical output when an electric current is injected thereinto.
 24. Thegallium nitride semiconductor laser device according to claim 16,wherein a layer disposed at a foot of the ridge structure has a layerthickness of 0.05 μm to 0.5 μm.
 25. The gallium nitride semiconductorlaser device according to claim 16, further comprising an insulatingfilm formed over side faces of the ridge and over surfaces which arelocated on both sides of the ridge and from which the ridge isprojected.
 26. The gallium nitride semiconductor laser device accordingto claim 25, further comprising an electrode formed on an upper surfaceof the ridge and over the insulating film.